Yeast Stress Response and Fermentation Efficiency: How to Survive the Making of Wine - A Review

Fermentation predictability and wine quality are directly dependent on wine yeast attributes that assist in the rapid establishment of numerical dominance in the early phase of wine fermentation, and that determine the ability to conduct an even and efficient fermentation to obtain a desirable alcohol degree. It is therefore not surprising that the primary selection criteria applied to most wine yeast strain development programmes relate to the overallobjective of achieving an efficient conversion of grape sugar to alcohol and carbon dioxide, at a controlled rate and without the development of off-flavours. Numerous factors influence the fermentation performance of wine yeast.  Following a successful inoculation of grape must with an appropriate starter culture strain, the ability of a wine yeast to adapt to and cope with the hostile environment and stress conditions prevailing in grape juice fermentation are of vital importance to fermentation performance. There is a direct correlation between fermentation efficiency and stress resistance, which refers to the ability of a yeast strain to adapt efficiently to a changing environment and unfavourable growth conditions. Successful yeast cellular adaptation to changes in extracellular parameters during wine fermentation requires the timely perception (sensing) of chemical or physical environmental parameters, followed by accurate transmission of the information to the relevant compartments of the cell.  Chemical parameters perceived during wine fermentation include the availability/concentration of certain nutrients (e.g., fermentable sugars, assimilable nitrogen, oxygen, vitamins, minerals, ergosterol and unsaturated fatty acids) and the presence of inhibitory substances (e.g., ethanol, acetic acid, fatty acids, sulfite, phenolic phytoalexins, mycotoxins, bacterial toxins and agrochemical residues). Signals of a physical nature include temperature, pH, agitation and osmotic pressure. The sensing of these environmental signals is carried out by specific receptor proteins, most of them situated on the cellular surface. Once perceived, the information is transmitted by a network of dedicated, interconnected signal transduction pathways to the relevant cellular compartments which implement theadaptive response, a process referred to as "stress response". Intensive research has focused on elucidating the molecular mechanisms involved in stress responses, which are evolutionarily well conserved. Besides furthering our understanding of the fundamental strategies for adaptation to hostile, industrial environments, and the biological resilience of Saccharomyces cerevisiae, the data are of key importance to the future improvement of wine yeast strains. This review describes the different types of stress experienced by wine yeast cells during their life cycles, summarises our current knowledge of some of the most important molecular processes required for the survival of the yeast cell, and highlights the potential benefits for future yeast strain development which can be derived from this research

Wine Science in the Omics Era: The Impact of Systems Biology on the Future of Wine Research

Industrial wine making confronts viticulturalists, wine makers, process engineers and scientists alike with a bewilderingarray of independent and semi-independent parameters that can in many cases only be optimized by trial and error.Furthermore, as most parameters are outside of individual control, predictability and consistency of the end productremain difficult to achieve. The traditional wine sciences of viticulture and oenology have been accumulating data setsand generating knowledge and know-how that has resulted in a significant optimization of the vine growing and winemaking processes. However, much of these processes remain based on empirical and even anecdotal evidence, andonly a small part of all the interactions and cause-effect relationships between individual input and output parametersis scientifically well understood. Indeed, the complexity of the process has prevented a deeper understanding of suchinteractions and causal relationships. New technologies and methods in the biological and chemical sciences, combinedwith improved tools of multivariate data analysis, open new opportunities to assess the entire vine growing and winemaking process from a more holistic perspective. This review outlines the current efforts to use the tools of systemsbiology in particular to better understand complex industrial processes such as wine making

Organic Acid Metabolism and the Impact of Fermentation Practices on Wine Acidity - A Review

The conversion of grape sugar to ethanol and carbon dioxide is the primary biochemical reaction in alcoholic wine fermentation, but microbial interactions, as well as complex secondary metabolic reactions, are equally relevant in terms of the composition of the final wine produced. The chemical composition of a wine determines the taste, flavour and aroma of the product, and is determined by many factors such as grape variety, geographical and viticultural conditions, microbial ecology of the grapes and of the fermentation processes, and the winemaking practices. Through the years, major advances have been made in understanding the biochemistry, ecology, physiology and molecular biology of the variousyeast strains involved in wine production, and how these yeasts affect wine chemistry and wine sensory properties. However, many important aspects of the impact of yeast on specific wine-relevant sensoryparameters remain little understood. One of these areas of limited knowledge is the contribution of individual wine yeast strains to the total organic acid profile of wine. Wine quality is indeed very directly linked to what wine tasters frequently refer to as the sugar–acid balance. The total acidity of a wine is therefore of prime sensory importance, and acidity adjustments are a frequent and legal practice in many wineries. However, the total acidity is the result of the sum of all the individual organic acids that are present in wine. Importantly, each of these acids has its own sensory attributes, with descriptors ranging from fresh to sour to metallic. It is therefore important not only to consider total acidity, but also thecontribution of each individual acid to the overall acid profile of the wine. This review will summarise the current knowledge about the origin, synthesis and analysis of organic acids in wine, as well as on themanagement of wine acidity

The Impact of Changes in Environmental Conditions on Organic Acid Production by Commercial Wine Yeast Strains

Acidity is one of the primary sensory elements in wine, and the balance of sugar and acidity is probably the strongest element affecting wine appreciation. However, little is known about how yeast strains and fermentation conditions will affect the production of fermentation-derived acids, including acetic, succinic and pyruvic acid. This study employs a multifactorial experimental design to provide a better understanding of how individual or simultaneous changes in environmental parameters such as pH, sugar and temperature influence the production of individual organic acids during fermentation in several yeast strains in synthetic must. Certain changes in environmental factors led to conserved trends between strains and treatments. Strains produced higher succinic acid levels when temperature was increased. Significant strain-dependent differences were observed when sugar concentrations were varied for both strains: the combinatorial impact of high initial sugars and fermentation temperature was more pronounced when increased pyruvic acid production was observed in yeast strain VIN13. On the other hand, while combinatorial influences are evident, higher sugar fermentation settings were largely characterised by high acetic acid concentrations for both strains. It is clear that simultaneous changes in sugar, pH andtemperature affect organic acid trends in a variable manner, depending on the particular combination of environmental parameters and yeast strain. The study provides valuable information regarding the mannerin which initial must parameters and environmental conditions throughout fermentation may affect wine acidity. Since many of these parameters can be controlled at least in part during the winemaking process,the data provide important background information for oenological strategies that aim to optimise the acid balance of wines

The Diversity and Dynamics of Indigenous Yeast Communities in Grape Must from Vineyards Employing Different Agronomic Practices and their Influence on Wine Fermentation

The current study evaluated the diversity of yeast species in Cabernet Sauvignon grape must derived from three neighbouring vineyards from a similar terroir but on which significantly different management practices are employed. The fermentation kinetics and yeast population dynamics were monitored from the beginning to the end of spontaneous fermentation. The grape musts were characterised by distinct yeast populations comprising oxidative, weakly fermentative and strongly fermentative yeasts. Different combinations of dominant non-Saccharomyces yeasts were observed in each must, with significantly different assortments of dominant species, including Starmerella bacillaris (synonym Candida zemplinina), Lachancea thermotolerans, Hanseniaspora uvarum, Candida parapsilosis and Wickerhamomyces anomalus. None of these yeast consortia appeared to affect the growth of Saccharomyces cerevisiae or inhibit the overall progress of fermentation. However, the percentage of fermentative yeasts was positively correlated with the fermentation rate. Glucose and fructose consumption rates suggested active participation of both glucophilic and fructophilic yeasts from the onset of fermentation. The data highlight two parameters, viz. initial cell concentration and yeast community composition, as important fermentation drivers and open the possibility to predict fermentation behaviour based on the initial composition of the yeast community

Understanding Problem Fermentations – A Review

Despite advances in winemaking technology and improvements in fermentation control, problem alcoholic andmalolactic fermentations remain a major oenological concern worldwide. This is due to possible depreciation ofproduct quality and its consequent negative economic impact. Various factors have been identified and studied overthe years, yet the occurrence of fermentation problems persists. The synergistic effect of the various factors amongsteach other provides additional challenges for the study of such fermentations. This literature review summarisesthe most frequently studied causes of problematic alcoholic and malolactic fermentations and in addition provides asummary of established and some potential new analytical technologies to monitor and investigate the phenomenonof stuck and sluggish fermentations

Determining the Impact of Industrial Wine Yeast Strains on Organic Acid Production Under White and Red Wine-like Fermentation Conditions

Organic acids are a major contributor to wine flavour and aroma. In the past, the scientific focus hasmostly been on organic acids derived from grapes or on the transformation of malic acid to lactic acid bylactic acid bacteria, since these acids contribute significantly to the final total acidity of wine. However,the organic acid concentration and composition also change significantly during alcoholic fermentation,yet only limited information regarding the impact of different yeast strains on these changes has beenpublished. Here we report on changes in organic acid (malic, tartaric, citric, succinic, acetic and pyruvic)composition during fermentation by five widely used industrial wine yeast strains in a synthetic grape must(MS300) reflecting two very different, but both wine-like, fermentation conditions. Samples were obtainedfrom three physiological stages during fermentation, namely the exponential growth phase (day 2), earlystationary phase (day 5) and late stationary phase (day 14). These different stages were selected to providemore information on acid evolution throughout fermentation, as well as on the impact of nutritional andenvironmental conditions during aerobic and anaerobic fermentation. Among other observations, somestrains (such as VIN13 and 285) were shown to be generally higher producers of most acids in white and/or red wine fermentation settings, while other strains (such as DV10) were generally lower acid producers.The data clearly demonstrate that different strains have different acid consumption and productionpatterns, and this presents a first step towards enabling winemakers to appropriately select strains foracid management during fermentation

Environmental Stress and Aroma Production During Wine Fermentation

The sensory description of wine uses the widest range of descriptive terminology of all food products, reflecting the complex nature of a product whose character depends on the balance of hundreds of individual flavour-active compounds. There are many tools that can influence flavour profiles or wine styles, one of which is the choice of a specific yeast strain. Yeasts contribute to wine flavour by producing volatile metabolites with different flavour profiles. The impact of changing environmental conditions on the production of flavour compounds by yeast strains remains largely unexplored. This is the first study investigating the impact of two mild fermentation stresses, hyperosmotic and temperature stress, on aroma production in synthetic must by commercial Saccharomyces cerevisiae wine strains. Hyperosmotic stress was imposed by cultivation of the yeast for 21 days in the must containing either 0.3 or 0.5 M sorbitol. The transient temperature stresses were applied for 16 h at 8° or 37°C for either two or eight days after commencement of the fermentation. Greater glycerol and acetic acid levels were produced by most yeasts when only hyperosmotic stress was applied. Hyperosmotic and temperature stress conditions produced a limited number of significant changes to the profile of the esters, higher alcohols and volatile fatty acids. These changes differed significantly for each strain and stress treatment, suggesting that the fermentation conditions can significantly alter the aromatic profile of a wine, although these stress impacts cannot be predicted in general. The changes to the aromatic profile are specific to each individual wine yeast strain

Corrigendum: Investigating the Effect of Selected Non-Saccharomyces Species on Wine Ecosystem Function and Major Volatiles [Front. Bioeng. Biotechnol., 6, (2018) (169)] DOI: 10.3389/fbioe.2018.00169

In the original article, there was a mistake in Figure 2 as published. The order of the graphs (A-H) is incorrect and does not match the caption nor the in-text citation. The corrected Figure 2 appears below. The authors apologize for this error and state that this does not change the scientific conclusions of the article in any way. The original article has been updated

Microbial diversity in grape musts from Austrian and South African grape varieties and regions

Vitis vinifera is one of the most widely planted crops and holds important economic value in South Africa and Austria. Grapes obtained from this plant harbour a complex fungal community which plays a crucial role in the wine fermentation process and influences wine flavour and aroma. For many years the contributions of the natural yeasts has been eclipsed by the use of active dry yeast (ADY) inoculant, mainly of the species Saccharomyces cerevisiae. However, recent studies show a growing interest in deciphering the natural microbial diversity and in promoting its persistence during fermentation in order to enhance wine typicity. The current preliminary study aims to provide a first broad assessment of the fungal community fingerprint of different grape varietals from different wine producing areas in Austria and South Africa through Automated Ribosomal Intergenic Spacer Analysis (ARISA). The ARISA profiles separated the samples according to country of origin, and suggested some regional and varietal separation within each country. Future work will evaluate the contribution of these fungal communities to wine chemical composition and sensorial distinctness